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© Retrotec 2017
Cannot Reach 25 Pa Factors
Rather than using the equation in the situation where the 25 Pa test pressure cannot be achieved, you can
use tables with the factors already calculated and the following procedure. Take a flow (CFM) reading at
the pressure achieved and multiply by the "Cannot reach 25 Pa factor
”
from Table 11 to get an estimate of
the flow that would result at 25 Pa.
For example: if you read 600 CFM at a building pressure of 19 Pa, the "Cannot reach 25 Pa factor" from the table is
1.18. Make the following calculation:
600 𝐶𝐹𝑀 ×1.18 = 708 𝐶𝐹𝑀
Thus the estimated result for CFM at 25 Pa will be 708 CFM.
The factors depend on the
n
value chosen, and Table 13 is for
n
of 0.6 which is typical for duct systems.
Table 11: Factors for when a building pressure of 25 Pa cannot be reached (n value of 0.6)
Pressure
achieved
Cannot reach
25 factor
25
1.00
24
1.02
23
1.05
22
1.08
21
1.11
20
1.14
19
1.18
18
1.22
17
1.26
16
1.31
15
1.36
14
1.42
13
1.48
12
1.55
11
1.64
10
1.73
9
1.85
8
1.98
7
2.15
6
2.35
5
2.63
